Optimize Bytes#hash and keep slow Slice(T)#hash

spec/std/slice_spec.cr

@@ -378,6 +378,14 @@ describe "Slice" do
     slice = Bytes[1, 2, 3, read_only: true]
     expect_raises { slice[0] = 0_u8 }
   end
+
+  it "hashes each item in collection" do
+    Slice[1, 2, 3].hash.should eq(Slice[1_u64, 2_u64, 3_u64].hash)
+  end
+
+  it "optimizes hash for Bytes" do
+    Bytes[1, 2, 3].hash.should_not eq(Slice[1, 2, 3].hash)
+  end
 end
 
 private def itself(*args)

src/crystal/hasher.cr

@@ -17,8 +17,8 @@ struct Crystal::Hasher
   # disclose the result of the hashes, we don't need the cryptographically
   # verified siphash-2-4, but can use the faster siphash-1-3 alternative.
   #
-  # On 32-bit systems, we prefer the halfsiphash-1-3 alternative (32-bit hashes)
-  # that should perform better than siphash-1-3 (64-bit hashes).
+  # On 32-bit systems, we prefer the halfsiphash-1-3 alternative (32-bit
+  # computations) that performs better than siphash-1-3 (64-bit computations).
 
   # TODO: use flag?(:bits64) for Crystal > 0.23.1
   {% if flag?(:x86_64) || flag?(:aarch64) %}
@@ -80,6 +80,11 @@ struct Crystal::Hasher
     self
   end
 
+  def slice(value : Bytes)
+    @siphash.update(value)
+    self
+  end
+
   def class(value)
     value.crystal_type_id.hash(self)
   end

src/slice.cr

@@ -495,6 +495,15 @@ struct Slice(T)
     nil
   end
 
+  # See `Object#hash(hasher)`
+  def hash(hasher)
+    {% if T == UInt8 %}
+      hasher.slice(self)
+    {% else %}
+      super hasher
+    {% end %}
+  end
+
   protected def check_writable
     raise "Can't write to read-only Slice" if @read_only
   end